The present invention relates to an apparatus and a process for manufacturing telecommunication cable in which a cylindrical core having one or more channels in its periphery is twisted and optical fibers are inserted into the resulting helical channels.
The large information transmission capacity of optical fibers has prompted their use in a wide variety of telecommunication applications having different physical and environmental conditions. Of particular importance are long-distance applications which can involve submarine, underground or overhead cable installations. Although individual optical fibers can be quite strong, having breaking strengths as great as 400-800 KPSI, the fibers require protection by cable structures that isolate them from tensile stresses which occur during installation and use. Also important is the protection the cable affords the fibers from the elements, particularly water.
A suitable cable structure which provides such protection for the optical fibers is described in detail and claimed in the co-pending related application mentioned above. Generally, as shown in partial cross-section in FIG. 1a, that cable comprises a central cylindrical core 1 having one or more helical channels 2 in its periphery (two are shown in the figure). The optical fibers 3 are randomly positioned within buffer tubes 5 that are located in the channels, with the core and fibers being overwrapped by a tape layer 4. Not shown in FIG. 1a is a serving of wires which are wrapped around the taped core to provide the tensile strength and other capabilities which may be required by the particular cable application. The controlled helical pitch of the channels, the inside diameter of the tubes, the ratio of the linear fiber length to the linear tube length, and the diameter of the fiber or tube helix combine to create a cable with a large elongation window, i.e., a cable for which a large cable strain produces little or no added fiber strain. Insufficient fiber- to tube-length ratios can allow stress to be transmitted to the fibers with possible breakage during installation and/or use, while excessive fiber- to tube-length ratios can cause greater optical transmission losses due to fiber bends in the tubes.
The very insertion of the fibers into the channels subjects them to undesirable stress which can affect their transmission performance. Some insertion stresses are nearly unavoidable, such as those generated from bends and twists which are imposed on the fibers as they pass from supply spools through a cabling machine into the channels. Additional sources of fiber stress are caused by differences between the feed rate of the core or helical pitch of the channels and the supply rate of the fibers.
Various devices for laying optical fibers in the channels of a central cylindrical support have been disclosed, for example U.S. Pat. Nos. 4,154,049, 4,205,899, 4,309,864, 4,388,799, 4,395,869, 4,411,130 and 4,497,164. Separate cabling machines sequentially perform the tasks of twisting the core to form helical channels and inserting the fibers into the channels; as a result they can subject the fibers to the additional insertion stresses noted above. To minimize these excess stresses, the cabling devices disclosed in these patents have therefore incorporated various approaches including servomechanisms and other feedback devices to synchronize the means for laying the fibers into the channels with the helical pitch. Other approaches involve the use of fiber insertion heads having flexible tubes which extend into and contact the channels and thus guide the fibers into the channels. Besides the relatively increased complexity and higher cost of the cabling machines disclosed in the patents mentioned above, insertion stresses are not completely eliminated.